The present invention relates to an electrochemical energy generating apparatus and an operating method thereof, and an electrochemical device forming the apparatus.
There are energy density and power density as quantities indicating characteristics of a cell. Energy density is an energy storage quantity per unit mass of the cell, and power density is a power quantity per unit mass of the cell. A lithium-ion secondary battery is often used as a power source for mobile devices because the lithium-ion secondary battery combines two features of a relatively high energy density and a very high power density, and also has reached a high degree of perfection. Recently, however, as the functionality of the mobile device has become higher, power consumption of the mobile device has tended to be increased, so that the lithium-ion secondary battery is desired to be further improved in energy density and power density.
Solutions to this would include the changing of electrode materials for a positive electrode/negative electrode, improvements in a method of coating electrode materials, improvements in a method of sealing in electrode materials, and the like, and research is being conducted to improve the energy density of the lithium-ion secondary battery. However, there are still high hurdles to clear for practical use. In addition, unless component materials used in the present lithium-ion secondary battery are changed, substantial improvements in energy density cannot be expected.
Thus, it is urgently necessary to develop a battery having higher energy density in place of the lithium-ion secondary battery, and a fuel cell is considered to be promising as one of candidates for a battery having higher energy density.
The fuel cell includes an anode electrode and a cathode electrode. A fuel is supplied to the anode electrode side, and an air or oxygen is supplied to the cathode electrode side. As a result, an oxidation-reduction reaction in which the fuel is oxidized by oxygen occurs on the anode electrode and the cathode electrode, and a part of chemical energy possessed by the fuel is converted into electric energy, which is then extracted.
Various kinds of fuel cells have already been proposed and produced on a trial basis, and a part of the various kinds of fuel cells have already been manufactured and put to practical use. According to an electrolyte used, these fuel cells are classified into alkaline fuel cells (AFCs), phosphoric acid fuel cells (PAFCs), molten carbonate fuel cells (MCFCs), solid oxide fuel cells (SOFCs), polymer electrolyte fuel cells (PEFCs), and the like. Of the polymer electrolyte fuel cells (PEFCs), direct methanol fuel cells (DMFCs), in which methanol as a fuel is directly supplied to an anode, are most likely to be used as an energy source for mobile devices, electric vehicles and the like among various fuel cells.
In a DMFC, a fuel of methanol is generally supplied as an aqueous solution of low concentration or high concentration to an anode side, and oxidized to carbon dioxide in a catalyst layer on the anode side. Protons generated at this time travel through an electrolyte membrane separating an anode and a cathode to the cathode, and then react with oxygen to form water on the cathode side. Reactions occurring at the anode, the cathode, and the DMFC as a whole are each expressed by the following equation.Anode: CH3OH+H2O→CO2+6e−+6H+Cathode: (3/2)O2+6e−+6H+→3H2ODMFC as a whole: CH3OH+(3/2)O2→CO2+2H2O
The energy density of methanol as the fuel of the DMFC is theoretically 4.8 kW/L, which is 10 times higher than the energy density of the lithium-ion secondary battery. That is, the fuel cell using methanol as a fuel has a potential to surpass the lithium-ion secondary battery in energy density. In addition, the DMFC eliminates a need for a reformer for extracting hydrogen from a fuel, and thus has an advantage of having a simple constitution. Further, polymer electrolyte fuel cells (PEFCs) such as the DMFC have an advantage of being able to operate at a lower temperature of 30° C. to 130° C. than other fuel cells.
However, the DMFC has a problem in that though theoretical voltage is 1.23 V, output voltage when power is actually generated by the DMFC is lowered to about 0.6 V or lower. A cause of the lowering of the output voltage is a voltage drop caused by internal resistances of the DMFC. Inside the DMFC, there are internal resistances such as a resistance involved in reactions occurring at both electrodes, a resistance involved in movement of substances, a resistance occurring when protons travel through the electrolyte membrane, a contact resistance, and the like. Energy that can be actually derived as electric energy from the oxidation of methanol is expressed by a product of the output voltage at the time of power generation and a quantity of electricity flowing through a circuit. Thus, when the output voltage at the time of power generation is lowered, the energy that can be actually derived is correspondingly decreased. Incidentally, the quantity of electricity that can be taken out to the circuit by the oxidation of methanol is proportional to a quantity of methanol within the DMFC when the whole quantity of methanol is oxidized at the anode according to the above-described equation.
The DMFC has another problem of methanol crossover. Methanol crossover is a phenomenon in which methanol passes through the electrolyte membrane from the anode side and reaches the cathode side due to two mechanisms of a phenomenon in which methanol is diffused and moved by a difference in methanol concentration between the anode side and the cathode side and an electroosmotic phenomenon in which hydrated methanol is carried by the movement of water, which is caused with the movement of protons.
When methanol crossover occurs, methanol that has passed through is oxidized on a catalyst on the cathode side. The methanol oxidation reaction on the cathode side is the same as the oxidation reaction on the anode side as described above, but causes a decrease in the output voltage of the DMFC (see “Fuel Cell Systems Explained”, Ohmsha, Ltd., p. 66). In addition, because methanol is not used for power generation on the anode side but is consumed on the cathode side, the quantity of electricity that can be taken out to the circuit is correspondingly reduced. Furthermore, because the catalyst on the cathode side is not a Pt—Ru alloy catalyst, but is a Pt catalyst, CO tends to be adsorbed on the surface of the catalyst, and catalyst poisoning occurs, for example.
As described above, the DMFC has two problems of a decrease in voltage, which decrease is caused by the internal resistance and methanol crossover, and consumption of fuel as a result of methanol crossover. These problems are a cause of a decrease in power generation efficiency of the DMFC. Accordingly, research and development for improving characteristics of materials forming the DMFC and research and development for optimizing operating conditions of the DMFC are being conducted to enhance the power generation efficiency of the DMFC.
In the research for improving characteristics of materials forming the DMFC, research into reducing methanol crossover, in particular, is actively pursued.
A method of using up all of methanol that has been supplied on the anode side, for example, is conceivable as a method for reducing methanol crossover. For this, it is necessary to improve catalytic activity per unit quantity on the anode side or increase a catalyst support quantity on the anode side. However, while research and development of new catalysts is being conducted, it is at present difficult to improve the activity of a Pt—Ru base catalyst generally used as a catalyst on the anode side, and an ideal catalyst having higher catalytic performance than that of the Pt—Ru base catalyst has not been found either. While increasing the catalyst support quantity can improve a rate of oxidation of methanol at the anode to a certain degree, increasing the catalyst raises the internal resistance, thus resulting in an adverse effect of lowering the output voltage.
In addition, research into electrolyte membranes that can minimize methanol crossover is actively pursued. However, a polyperfluoroalkylsulfonic acid base resin membrane (for example a Nafion (registered trademark) membrane), which is generally used as an electrolyte membrane for the DMFC, has an advantage of high proton conductivity, but has low capability to obstruct the penetration of methanol. Thus, high proton conductivity and high capability to obstruct the penetration of methanol tend to run counter to each other. An ideal catalyst has not yet been found, and an optimum electrolyte membrane has not yet been found either.
On the other hand, research and development is being conducted to improve the power generation characteristic of the DMFC by controlling operating conditions as another method than the improvement of characteristics of component materials. For example, Reference Document 1 (Journal of Power Sources, 112, 339 to 352 (2002)) has a description showing that the power density and power generation efficiency of the DMFC vary depending mainly on operating temperature, a fuel supply quantity, fuel concentration and the like, and describes changes in characteristics of the fuel cell when these operating conditions are changed.
In this Reference Document 1 (Journal of Power Sources, 112), data on the characteristics of the fuel cell in different operating conditions is collected with the temperature set at 40° C., 60° C., and 80° C., the fuel supply flow rate set at 0.15 mL/min, 0.5 mL/min, and 5 mL/min, and the fuel concentration set at 2 mol/L, 1 mol/L, and 0.5 mol/L. A result shows that a maximum power density is obtained when the fuel cell is made to generate power under operating conditions where the fuel cell is operated at 80° C., and the fuel concentration is set at the minimum of 0.5 mol/L and the fuel supply flow rate is set at the maximum of 5 mL/min. In addition, it is also described a fact that under the above operating conditions, the power density is high, but the power generation efficiency becomes poor because the methanol crossover is increased. It is also described a fact that even when the same output is obtained, the power generation efficiency varies greatly depending on operating conditions.
Accordingly, a method is conceivable which creates a database in advance in which operating conditions that increase the power generation efficiency are determined after the characteristics of the fuel cell are measured while finely changing parameters such as the temperature, the fuel supply flow rate, the fuel concentration, and the like, and sets operating conditions of the fuel cell on the basis of the database.
In addition, as a method for optimizing the operating conditions, Reference Document 2 (Japanese Patent Laid-Open No. 2003-22830) proposes a method in which fuel flow rate control means for controlling the quantity of the fuel supplied from a fuel tank according to the concentration of the fuel is used to improve the performance of the fuel cell. At this time, the internal model of the DMFC is represented by a mathematical formula, and thereby power generation conditions are expressed in the form of a formula to determine the operating conditions.
However, the method which creates a database and performs control on the basis of the database has a disadvantage in that there are too many parameters affecting the power generation efficiency of the DMFC and thus the measurement takes time. The method has another disadvantage in that the amount of measurement data becomes enormous, and thus a control program is also increased in size.
Further, it is known that the internal characteristics of the fuel cell change due to CO poisoning occurring at the anode side catalyst, flooding occurring at the cathode, degradation of the electrolyte membrane, and the like. When power is actually generated by using the DMFC, it is assumed that the DMFC is used for at least a few months to one year or more. Meanwhile, the internal characteristics of the fuel cell change inevitably, and operating conditions that maximize the power generation efficiency of the DMFC change from moment to moment.
Methods as described in Patent Document 1 (Japanese Patent No. 3451111 (pages 3 to 5, FIGS. 1 to 8)) and Patent Document 2 (Japanese Patent Laid-Open No. 2001-338667 (pages 4 to 6, FIGS. 1 to 3)) determine operating conditions on the basis of data measured in advance or a mathematical formula created in advance, and thus does not take changes in the internal characteristics of the fuel cell into consideration and cannot deal with the changes in the internal characteristics. The control methods that make the DMFC generate power under the operating conditions derived from the rule thus determined in advance does not provide the power generation characteristic as expected during the actual operation of the DMFC, and cannot make the DMFC continue generating power under the operating conditions where the power generation efficiency is high for a long period of time. As a result, electric energy cannot be derived from methanol efficiently. Even when methanol having high energy density is used, the advantages of methanol cannot be utilized. Thus, only an energy density equal to or lower than that of the lithium-ion secondary battery is obtained.
In order to solve such a problem, Patent Document 1 (Japanese Patent No. 3451111) and Patent Document 2 (Japanese Patent Laid-Open No. 2001-338667) have proposed a method for controlling the fuel cell which method uses, for control, a reference electrode that has conventionally been used for purposes of studying causes of degradation in performance of the fuel cell and degradation mechanisms.
Patent Document 1 (Japanese Patent No. 3451111) proposes a control method for properly maintaining the water-retaining states of a polymer electrolyte membrane and electrodes, which method detects the potentials of an anode and a cathode using a reference electrode, determines whether water content inside the fuel cell is high or low on the basis of the potentials, and adjusts the flow rates of a hydrogen containing gas and an oxygen containing gas supplied to the anode and the cathode and amounts of humidification of these gases.
Patent Document 2 (Japanese Patent Laid-Open No. 2001-338667) proposes a fuel cell control system that has a reference electrode formed by a reversible hydrogen electrode in the vicinity of an end part of at least one of a fuel electrode and an air electrode that are disposed on both sides of an electrolyte, detects a potential difference between the reference electrode and the fuel electrode or a potential difference between the reference electrode and the air electrode, quickly determines an operating parameter indicating highest energy conversion efficiency on the basis of a resulting detection signal, and operates the fuel cell in optimum conditions at all times.